Many aero-engines adopt a dovetail style of fan blade root which locates in a corresponding slot formed in the rim of the fan disc. During service operation, the fan assembly is subject to a complex loading system, consisting of centripetal load, gas-bending and vibration. The dovetail geometry copes particularly well with this kind of loading conditions. Retention devices are fitted to restrain axial movement of fan blades resisting thrust loading under normal running and axial loading during fan blade impact events.
Engine casings must be capable of containing the release of a single compressor or turbine blade, or any likely combinations of blades. In particular, an engine must pass a fan blade-off test to demonstrate mechanical integrity of all systems following the loss of a fan blade. The test is a single-shot exercise, comprising the deliberate release of the portion of a blade outboard of its retention feature at the maximum low pressure shaft speed, either on a full engine or a-fan-blade-off rig.
When the blade is released, it is retained by the casing and is then hit by the following blade, which tends to push the released blade backward (toward the rear of the engine). In reaction, it produces a force pushing the following blade (still retained by the fan disc) forward. The resulting load can be as much as about 80,000 lbf (356 kN) in the axial direction.
Bird impacts on fan blades can also cause axial high loads.
The retention device restraining axial movement of a fan blade must be able to withstand these types of axial load. However, it should also be as light as possible to reduce the weight of the engine.
Shear keys (see e.g. U.S. Pat. No. 5,624,233) and thrust rings (see e.g. GB A 2262139) can be used as retention devices. Another type of retention device takes the form of individual retainer or shear plates positioned at the ends of the fan disc slots.